The present invention relates to semiconductor type gas sensors having two terminals and comprising a heater and a gas sensitive semiconductor covering the heater.
Gas sensors heretofore known include those of the type having a four-terminal structure wherein a heater circuit in the form of a platinum film or the like formed by vapor deposition and a gas sensing semiconductor circuit made of SnO.sub.2 or like metal oxide are electrically insulated from each other by an insulation plate, and each of the heater and the semiconductor has a pair of terminals at its opposite ends. Gas sensors of another type are also known which comprise such a heater circuit and gas sensing semiconductor circuit connected together each at one end and which have three terminals, i.e., one at the connection and two at the other ends of the circuits.
These two types of gas sensors have the following drawback. When such gas sensors are energized in clean air, current passes through the heater and the gas sensing semiconductor. The heater develops heat according to Joule's law, heating the semiconductor, while the semiconductor itself also evolves heat owing to the passage of current therethrough. Thus the semiconductor is maintained at a given high temperature to exhibit a definite standard resistance. However, the amount of heat developed by the heater and the amount of heat evolved by the current through the semiconductor itself are not independent but influence each other in a complicated fashion. If the amount of heat developed by the heater is altered, for example, by variations in the voltage of the heater power source, the temperature of the semiconductor varies. Since the electric resistance of the semiconductor also varies with its temperature, the standard resistance then differs considerably from the normal value, with the result that the intensity of the output signal emitted by the gas sensor differs from the usual level even at the same gas concentration. On the other hand, a variation in the ambient temperature or humidity to which the gas sensor is exposed of course alters the standard resistance of the semiconductor. This further varies the temperature of the heater to change the amount of Joule heat produced by the heater, consequently greatly changing the resistance of the semiconductor.